NIST-7

File:Atomic Clock024.jpg

NIST-7 was the atomic clock used by the United States from 1993 to 1999. It was one of a series of Atomic Clocks {{Cite web|title = A Brief History of Atomic Clocks at NIST|url = http://tf.nist.gov/cesium/atomichistory.htm|website = tf.nist.gov|access-date = 2016-01-25}} at the National Institute of Standards and Technology. Eventually, it achieved an uncertainty of 5 × 10⁻¹⁵. The caesium beam clock served as the nation's primary time and frequency standard during that time period, but it has since been replaced with the more accurate NIST-F1, a caesium fountain{{Cite web|title = How Does the NIST-7 Atomic Clock Work? {{!}} Time and Navigation|url = https://timeandnavigation.si.edu/multimedia-asset/how-does-the-nist-7-atomic-clock-work|website = timeandnavigation.si.edu|access-date = 2016-01-25}} atomic clock that neither gains nor loses one second in 100 million years.